Various types of acoustic flowmeters are known, one of the most versatile being an ultrasonic flowmeter of the transmission, or contra-propagating transit-time differential type. These may be formed in a variety of structural or mechanical arrangements, but all have the common feature that sonic energy, usually at ultrasonic frequencies, is transmitted through a fluid and received by a detector. Both direct transmission and reflective flow sensors are known. A transit-time differential flowmeter functions by determining the difference between the rate of propagation of the sound energy when flowing with and against the stream of fluid the flow rate of which is being determined. This is achieved by assessing the difference between the propagation time with and against the fluid flow, the technique has been used for some time. It has the advantage that besides being reliable and accurate, it can also be used to determine the velocity of sound in the fluid flowing in the flow tubes which, if appropriate, can be used to establish what fluid is present since the velocity of sound is different in different fluids. Industrial processes where the fluid may be changed from time to time benefit particularly from this feature. Although industrial applications are numerous, and transit time differential flowmeters have been used for a wide range of conditions from high temperatures to very low (cryogenic) temperatures and over a wide range of pressures, these usually involve a system in which the sound is transmitted at an angle across the flow tube from one side to the other.
Although it is common for the acoustic energy to be transmitted into the flow of the fluid being measured at an angle to the direction of travel of the fluid, and many flowmeters operate in this way, there are particular advantages in creating a substantially plane wavefront travelling substantially parallel to the direction of fluid flow. The most consistent and accurate results are achieved with a plane wavefront, and for this purpose one known such flowmeter has transducers at each end of a flow tube and inlet and outlet connectors for the fluid into the sides of the flow tube.
However, in order to generate a plane wavefront the wavelength must be greater than the diameter of the flow tube, which means that there is a limit to the upper frequency which can be used with any one flow tube diameter.
The use of acoustic flowmeters is becoming increasingly adopted in therapeutic and medical applications where measurement of the rate of flow of body fluids, especially blood, is of importance. The quantity of fluid flowing, and the flow rates, however, are rather small and it would be convenient to be able to use a small bore flow tube for such purposes. Typically, the smallest useful diameter for a flow tube of a conventional known flowmeter is in the region of 6 mm, whereas a flow tube having a diameter of 1 mm would be more appropriate for the conditions met in medical applications. This, however, is difficult to achieve, partly because of the difficulty in coupling the acoustic signal in to a fluid having such small transverse dimension at the frequencies of energisation of the transducer which must be used, and partly because such small tubes are difficult to clean, whereas absolute cleanliness, even sterility, is an essential requirement in medical situations.
As in known, the velocity of sound is different in different fluids, and the fact that the frequency of the sound signal is related to the velocity of sound as well as to the flow rate of fluid, means that very small transit time differentials are experienced if the flow rate is relatively low and the velocity of sound in the fluid is relatively high.